1. Field of the Invention
The present invention relates to dual-band RF (Radio Frequency) power amplifiers. More particularly, this invention relates to high efficiency RF power amplifiers having a switching circuit, which creates a low impedance at the second harmonic of each frequency band, for dual-band operation.
2. Description of the Related Art
In portable wireless communication systems, there is a strong need for power amplifiers with high efficiency to maximize the amount of talk time obtained from a power source, such as a battery. It is well known in the art of RF power amplifiers to consider both the fundamental frequency, and harmonic components of the fundamental frequency, for increasing the efficiency to an optimal level. In addition to obtaining impedance matching at the fundamental frequency, the efficiency is increased by conditions that provide zero impedance for even multiples (even harmonics) of the fundamental frequency. The following background documents are incorporated by reference herein in their entireties:
[1] D. M. Snider," A Theoretical Analysis and Experimental Confirmation of the Optimally Loaded and Overdriven RF Power Amplifier," IEEE Tran, Electron Devices, vol. ED-14, pp. 851-857, December 1967. PA0 [2] J. E. Mitzlaff, "High Efficiency RF Power Amplifier," U.S. Pat. No. 4,717,884, January 1988. PA0 [3] M. A. Khatibzadeh, "Monolithically Realizable Harmonic Trapping Circuit," U.S. Pat. No. 5,095,285, March 1992. PA0 [4] N. Furutani, et al., "High Efficiency RF Power Amplifier," U.S. Pat. No. 5,159,287, October 1992. PA0 [5] P. M. White, "Effect of Input Harmonic Terminations of High Efficiency Class-B and Class-F Operation of PHEMT Devices," IEEE MIT-S Dig., pp. 1611-1614, 1998. PA0 [6] M. Masahiro, et al., "Radio-Frequency Power Amplifier with Input Impedance Matching Circuit Based on Harmonic Wave," U.S. Pat. No. 5,592,122, January 1997. PA0 [7] A. Adar, "Multiple-Band Amplifier," U.S. Pat. No. 5,774,017, June 1998.
A Class F amplifier is well known as a device which operates primarily as a switch. For this reason, the power dissipation is lower and the stage efficiency is higher than for other amplifiers. Class F operation is characterized by limiting the voltage across the active device to approximately twice the supply voltage [reference 2]. Class F power amplifiers are a most popular design because they are known for high efficiency, wherein the impedance at even harmonic frequencies at the transistor output (drain or collector) is set to a short-circuit (low impedance), and the impedance at the odd harmonic frequencies at the transistor output is set to an open-circuit (high 8 infinite impedance) [references 1-4]. The derivation of the harmonic impedance of a Class F amplifier [reference 1] is based on a Class B biasing operation. In Class B operation, the current flows for only 180.degree. of the AC cycle, whereas in Class A operation, the transistor is active for 360.degree. of the AC cycle for a linear reproduction of the input. When the amplifier is biased at a Class AB state (which is a hybrid between Class A and Class B operation, i.e. the bias voltage is chosen so that current flows for more than half of the cycle for higher efficiency than Class A but does not provide a linear reproduction like Class A), the impedance of the even harmonics is still zero, but the impedance of the odd harmonics is no longer infinite.
In addition, it is well known that second harmonic frequency termination is a dominant factor in improving the efficiency of a power amplifier. Furthermore, providing the second harmonic termination at the transistor input (gate or base), in addition to the transistor output, may improve the overall efficiency significantly [references 5, 6].
FIG. 1 shows a circuit diagram of a conventional single band high efficiency power amplifier. The even harmonic resonator 1 provides zero impedance for the second harmonic, and the input and output fundamental matching network 2,3 provides the prescribed load impedance at the fundamental frequency to Field Effect Transistor (FET) 4. However, dual-band operation in portable units is becoming indispensable because of dual-band communication systems, such as GSM (Global System for Mobil Communications).
However, the conventional high efficiency power amplifier shown in FIG. 1 can not provide dual-band operation. Although two single band power amplifiers, each of which operate at a specific frequency band, can be used in a dual-band handset (e.g., a telephone), a single dual-band amplifier provides instant reductions in the costs of manufacture and allows for a reduction in the size of the respective device, and saves power.
In addition, in the dual-band GSM system, the second harmonic of the cellular frequency band (around 900 MHz) is within the fundamental PCS (Personal Communications System) frequency band (around 1800 MHz). These frequency values indicate that a high efficiency GSM dual-band power amplifier should provide a low impedance at around 1800 MHz at the transistor output (or/and input) under cellular-band operation, and provide a prescribed impedance at around 1800 MHz and a low impedance at around 3600 MHz under PCS-band operation. Thus, there is a need for a single amplifier having dual-band capabilities that overcomes the problems of the prior art.